This application for a K23 award contains two complementary components: a career development plan and a research plan. The career development plan will support critical additional training for the candidate, Dr. Steven DuBois, a pediatric oncologist at the University of California, San Francisco (UCSF). Dr. DuBois has used his previous training to develop a burgeoning developmental therapeutics research program focused on pediatric solid tumors. Given the increasing role of biomarkers in early phase clinical studies, Dr. DuBois now requires specific training in biomarker development and validation. He will use his flow cytometry-based marker of bone marrow micrometastatic disease in patients with Ewing sarcoma as a platform on which to obtain additional didactic and hands-on training in flow cytometry and phospho-flow cytometry. He will also complete dedicated training focused on other techniques for detecting micrometastatic disease, molecular markers in oncology, and pharmacogenomics. Dr. DuBois has assembled an outstanding team of multidisciplinary mentors to facilitate his training and research. Dr. Katherine Matthay, an international expert in clinical and translational studies in pediatric solid tumors, will serve as primary mentor. She will oversee the overall training and research plans, providing her wealth of experience in this field. Dr. Mignon Loh will provide her expertise on the use of phospho-flow cytometry to study intracellular signaling in cancer cells. Dr. Elizabeth Sinclair will provide her expertise in the use of flow cytometry to detect rare populations of cells. The research plan focuses on a flow cytometry-based technique for detecting occult tumor cells in the bone marrow of patients with Ewing sarcoma. Dr. DuBois developed this technique and his preliminary studies demonstrate that bone marrow micrometastatic disease is nearly universal in patients with Ewing sarcoma, though the number of tumor cells varies widely between patients. In Aim 1 of this application, he proposes to assess the magnitude of the association between progression-free survival and baseline bone marrow tumor cells in untreated patients with Ewing sarcoma. In Aim 2, he will determine the clearance rate of bone marrow tumor cells detected by flow cytometry in patients with Ewing sarcoma. In Aim 3, he will evaluate the ability of flow cytometry to interrogate insulin-like growth factor-1 receptor (IGF-1R) signaling pathways in occult bone marrow Ewing sarcoma cells. Incorporation of this assay into his current flow cytometry assay will allow a clinically-relevant signaling pathway to be studied in bone marrow tumor cells. The proposed studies will provide additional data needed to support an R01 application to study bone marrow micrometastatic disease as a tool for risk-stratification in a national phase III clinical trial for Ewing sarcoma.